Return actual availability for drivers, plus add limit in environment

With #88, the availability can be returned in order to process the queue accordingly. Although, that's still not quite implemented yet.

Based on each driver, we could do requests to their APIs (for example, in DigitalOcean driver, you could just request account info to API, look for the droplet limit, then do another request to count how many droplets are running) and pass `true` or `false` based on the availability. Feel free to PR your works on it if we don't work on it first.

Plus that, a limit for maximum spawned up portals can be set in the enviroment variables. I'll surely take this last one myself.